1. Field of the Invention
This invention pertains to manufacturing glass products from molten mineral material. More particularly, this invention pertains to controlling the flow and viscosity of molten glass during production.
2. Description of the Background Art
It is a common practice to manufacture glass by supplying molten mineral material from a furnace or melter to a glass forming apparatus. One of the problems which has long plagued the manufacturers of glass is the variation in the viscosity of the glass flowing into the glass forming apparatus. In a continuous glass manufacturing process, variations in the viscosity of the glass can adversely affect the consistency and quality of the end product.
Existing methods of glass viscosity measurement are not sufficient for adequate control. Typically, a glass sample is taken from the furnace or forehearth and the sample is removed to a testing station where the viscosity is determined using calibrated instruments. The process takes a considerable amount of time, and is done off line. The results of the viscosity analysis are often learned too late to make any correction in the fiber forming process.
Chrisman in U.S. Pat. No. 4,277,274, discloses a method of controlling a glass melting furnace in which the viscosity is determined, and in which molten glass is added to the forehearth in response to deviations from glass viscosity setpoints. The viscosity in Chrisman is determined using sensed electrical conductivity of the glass flowing through the forehearth. Chrisman also suggests that the viscosity can be determined using a strain gauge on the shaft of the mixer which can be positioned within the forehearth. These solutions proposed by Chrisman involve some uncertainty because of the fact that the glass on which the viscosity is being measured is still within the forehearth. It has been found that measuring viscosity with equipment installed in the mainstream flow of molten glass, such as in the forehearth, is difficult due to the hostile environment within the molten glass for the equipment. The lack of access to the equipment gives the inevitable result that the equipment fails and the measuring process is out of control.
Glass viscosity is one of the major parameters controlling production in the glass industry. Currently, the measurements of glass viscosity discussed above are not being done more than two of three times a day in continuous glass furnaces. This does not provide enough control for preventive actions to correct undesirable viscosities. The viscosity of glass depends on both temperature and composition. Viscosity changes can characterize continuous change in the composition of glass, providing the temperature of the glass in the measuring system is controlled.
There is still a need for a method and apparatus which enables the glass viscosity to be measured on line, and in a continuous mode. It is also important that the viscosity measurement method be accomplished without disturbing the glass manufacturing process. Further, the equipment should be outside of the molten glass in order to give access and longevity to the equipment.